Heretofore, PS plates having an oleophilic photosensitive resin layer formed on a hydrophilic support have been widely used for lithographic printing plate precursors. For making printing plates from them, generally employed is a method of mask-exposing (surface-exposing) the precursor via a lith film followed by dissolving and removing the non-image area to obtain a desired printing plate. Recently, digital computing technologies have become much popularized for electronically processing image information, and accumulating and outputting the processed data by the use of computers. Various new-type image outputting systems that correspond to such digital technologies have now been put into practical use. As a result, a computer-to-plate (CTP) technique is desired that comprises scanning high-directivity light such as laser light in accordance with digitalized image information data for direct production of printing plates not via lith films, and it is now an important technical theme to obtain lithographic printing plate precursors suitable to it.
For such scanning-exposable lithographic printing plate precursors, a structure has been proposed, comprising an oleophilic photosensitive resin layer (hereinafter referred to as photosensitive layer or recording layer) that contains a photosensitive compound capable of generating an active species such as radical or Broensted acid through laser exposure, formed on a hydrophilic support, and its commercial products are now available on the market. The lithographic printing plate precursor of the type is laser-scanned on the basis of digital information to generate the active species, then the recording layer therein undergoes physical or chemical changes owing to the effect of the active species, thereby having an insoluble area, and then this is developed to give a negative lithographic printing plate. In particular, the lithographic printing plate precursor of a type having, formed on a hydrophilic support thereof, a photopolymerizable recording layer that contains a high-performance photopolymerization initiator, an addition-polymerizable ethylenic unsaturated compound and an alkali developer-soluble binder polymer, and optionally oxygen-blocking protective layer has various advantages in that its producibility is high, it may be developed in a simplified manner, its resolution is high and the ink acceptability of the printing plate from it is good, and therefore the precursor may give a printing plate having desired printing capabilities.
Heretofore, for the binder polymer to constitute the recording layer, used are organic polymers soluble in alkali developer, such as methacrylic acid copolymers, acrylic acid copolymers, itaconic acid copolymers, crotonic acid copolymers, maleic acid copolymers, partially-esterified maleic acid copolymers (for example, see JP-B 59-44615, JP-B 54-34327, JP-B 58-12577, JP-B 54-25957, JP-A 54-92723, JP-A 59-53836, JP-A 59-71048, and JP-A 2002-40652). In lithographic printing plate precursors having a recording layer that contains such an ordinary binder polymer, the solubility in alkali of the binder polymer in the non-image area tends to be insufficient. In particular, carboxyl group-having polymers of good alkali solubility are favorably used from the viewpoint of rapid developability. However, since the carboxyl group contains an ester bond in the linking group moiety to the terminal carboxyl group (—COOH) in the polymer, and since the ester bond moiety is hydrolyzed with time in developer, the polymer loses the terminal carboxyl group that is an alkali-soluble site. As a result, the binder polymer solubility in developer lowers, and the binder polymer once dissolved in developer during development therein may deposit to form development sediment, and it may impart a great deal of load to printing systems, or the deposited development sediment may adhere to printing plates to cause stains in prints. To that effect, the polymer binders have many problems.